OK, I have news for you and it's not going to be what you want to hear. But Mark, please don't shoot the messenger.
Firstly, if you belong to a labour union, that union has the sole right and obligation to represent you against the employer. So if you believe that you've been disciplined unfairly, or simply improperly (such as bringing it up in the lunchroom) then you have to raise it with your union if you do anything at all.
Now, if the union stewards are all in with management, then report this to your steward and if nothing happens, go over the steward's head. Or get a few other co-workers who have also been ignored by your steward to all contact above the steward. No union official wants to risk a job site's members rejecting the union just because some steward, who is low in the hierarchy, is in the pocket of the employer.
You haven't been officially disciplined yet, so you can't bang the drum too loudly. But you should definitely let the steward know that you're not happy about how this is playing out. In fact, if the steward is in tight with management and you really show some teeth, then the steward might suggest to management that they back off this issue unless there's proof and the discipline is done by the book.
It's time to objective about the situation. You can't stop rumours from starting. You can't help what your co-workers think of you. So just take the moral high ground and press on. There's no point in worrying about things you cannot change.
Same thing with what the employer "might do". They might discipline you. They might fire you. They might do nothing. The only thing that might help is for you to talk to the steward and be serious, and to start advocating for tools to avoid false accusations, like cameras on the buses or something. You want to make yourself out to be the victim, but not with any whining or sympathy-gathering, rather in an assertive way. That might help, but there are no guarantees of course.
Does that assist, Mark? If there's more to discuss then please reply. If I've answered you then I'd appreciate a positive service rating please.